The circadian systems of vertebrates appear to be composed of several identified neural and endocrine structures and almost certainly others as yet unidentified. These include the pineal gland, suprachiasmatic nucleus (SCN) and perhaps the retina. Though some of its components have been identified we still know little about the overall physiological organization of the circadian system and its relationships with other control systems particularly that which regulates reproduction. We propose here to obtain action spectra for the phase shifting effect of light pulses, and for the effects of light on the reproductive system of golden hamsters. This will enable us to determine which photopigments are involved in these crucial responses and for the first time will provide the opportunity for real experimental control over the major variable used in experiments in this field, visible light. The hamster with its precise well-studied circadian locomotor rhythm and its dramatic photoperiodic reproductive response is an ideal model system for use in this work. We also propose to study the organization of the circadian system in birds. The avian pineal organ has been shown to be an autonomus circadian oscillator which is transplantable from one bird to another and oscillates in vitro. It is also clear that the SCN and retinae of birds are involved in circadian rhythmicity. We will measure and describe circadian rhythms of circulating melatonin in individual birds, determine if the melatonin is of pineal or retinal origin and ask whether its rhythmicity depends on or is influenced by the SCN. If we can abolish melatonin rhythmicity by pinealectomy, we will attempt to restore it with pineal transplants. Finally, we will attempt to restore behavioral locomotor rhythmicity (which is abolished by pinealectomy) by artificially producing a "circadian" rhythm of exogeneous melatonin in pinealectomized birds. If we are successful in this attempt, we will repeat the experiments on birds with SCN lesions to determine if the SCN is a target for pineal melatonin (or some other component of output pathway) or involved in control of rhythmicity as part of a separate parallel mechanism.